1. Field of the Art
The present invention relates in general to a resistor element having a thin electrically resistive film, and more particularly to such a resistor element which is suitably used as a detecting element for a temperature sensor or a thermal flow meter, for example.
1. Discussion of the Prior Art
A resistor element which uses a thin metallic film as an electrically resistive body is known. The resistor element is adapted to electrically detect the temperature of a fluid, for example, by utilizing temperature dependence of an electrical resistance of the metallic film. An example of the resistor element is disclosed in laid-open Publication No. 60-60521 of unexamined Japanese Patent Application. This resistor element includes an electrically insulating ceramic support on which the electrically resistive metallic film is formed, and a protective glass coating covering the outer surface of the metallic film. The resistor element of this type is suitably used as a detecting element for a temperature sensor or a thermal flow meter, for example.
The resistor element of the above type is often subjected to a considerably high temperature during use, due to heat generated by the element itself, or heat transferred from the surrounding atmosphere. In view of this situation, the electrically resistive film is formed of a metal such as platinum, rhodium, palladium, gold, silver and nickel, or an alloy thereof. This metallic film is applied to a bearing surface of the ceramic support, in a physical or chemical method such as sputtering, chemical vapor deposition (CVD), vacuum evaporation, and plating. The applied metallic film is then trimmed by a laser as needed, so that the metallic film is formed in a suitable pattern, e.g. in a spiral or a zigzag pattern. The thus formed film has a predetermined value of electrical resistance.
The metallic film is made of a material which is unlikely to be oxidized, while the glass coating formed on the outer surface of the metallic film is formed of an oxide, whereby the glass coating has a relatively low affinity for the metallic film. When a glass material is heat-treated or fused to form the glass coating on the metallic film, the fused glass tends to be repelled from the metallic film because of the low affinity therebetween. As a result, the glass coating is not able to cover the entire area of the outer surface of the metallic film.
Further, when the metallic film is formed on the ceramic support, air may be trapped between the film and the ceramic support, resulting in poor adhesion of the film to the bearing surface of the support. In addition, the metallic film made of the above-described material also has a relatively low affinity for the ceramic support formed of an oxide such as alumina, and therefore suffers from a low degree of bonding strength with respect to the ceramic support. As stated above, the glass coating formed on the outer surface of the metallic film is also formed of an oxide, and has a relatively low affinity for the metallic film. Accordingly, it has been rather difficult to assure sufficient bonding strength between the glass coating and the metallic film.
In the resistor element as described above, the metallic film and the glass coating suffer from cracks, or separation or flake-off from the ceramic support or the metallic film, due to thermal stresses caused by repetitive temperature changes during use of the element. Thus, the conventional resistor element is not satisfactory in its durability.